1. Field of the Invention
This invention relates to scrubbing of chlorine. More particularly, it relates to catalytic decomposition of hypochlorite formed by scrubbing of chlorine-containing gas.
2. Description of the Prior Art
Scrubbing of chlorine-containing gases with alkali or alkaline earth metal hydroxide solution eliminates discharge to the atmosphere of most of the chlorine. However, the principal product of such scrubbing, hypochlorite, is often present in sufficiently high concentration to contaminate or pollute and create an objectionable odor in the streams or ponds of water receiving it.
It has previously been proposed to decompose such hypochlorites by exposing them to metal oxides such as the oxides of cobalt, copper, nickel or the like.
Kriegsheim U.S. Pat. No. 1,153,502 suggests, however, that the speed and completeness of the action depends very materially on the physical form of the oxide and upon the circumstances such as upon the way in which the reaction mixture and the catalyst are brought together. Kriegsheim, therefore, suggested that the salts of cobalt and other metals should be reacted with a zeolite, apparently to form a compound catalyst. Whether this compound catalyst included the metal as a salt or in oxide form is unclear.
Vasilev and Mikhaylova in KINETICS OF CATALYTIC DECOMPOSITION OF SODIUM HYPOCHLORITE (Kum vuprosa za kinetikata na katalitichnoto razlagane na natriev khipokhlorit.) Godishnik na Khimiko- Tekhologicheskiya Institut, Vol. 10, No. 2, pp. 25-32, 1963, discussed the use of copper, cobalt and nickel catalysts using chloride salts of these metals. They concluded that cobalt was the most effective catalyst. However, conversion of the amount of catalyst used (1 gram-mole per liter) to ppm indicates that a huge amount of catalyst (over 56,000 ppm) was used which would, of course, be economically unattractive. The recital of the amount of other ingredients indicated that Vasilev et al were operating in a pH range of about 13.2. SUMMARY OF THE INVENTION
After extended investigation, I have found that the problems outlined above can be substantially eliminated by catalytic decomposition of the hypochlorite into basically non-polluting products, chloride of the alkali metal or alkaline earth metal and oxygen. To do this, I employ, as catalyst a material containing one or more of the elements cobalt, nickel, copper and calcium, while operating in a pH range of 7-13. The catalyst concentration is at least 9 ppm and most advantageously is between 9-1000 ppm. Representative materials for supplying the catalyst, which appears to be converted to the oxide form in the course of the decomposition of the hypochlorite, include (1) salts (nonoxides) such as the nitrates and chlorides, for example, the hydrated form Co(NO.sub.3).sub.2.6H.sub.2 O for cobalt (the most effective catalyst according to the invention), (2) the fused metal, and (3) the metal powder, although decomposition rates are generally slower for the catalyst in elemental form.